From the very outset in 1970's of the commercial manufacture of hardmetal parts having wear-resistant coatings, the use of borides as layering materials has been desired, e.g., DE-OS 22 63 210. Specification DE-OS 25 25 185 disclosed an intermediate layer of one or more borides and an external layer of aluminum and/or zirconium oxide. The intermediate boride layers therein are deposited for improved adhesion of the oxide protective layer.
But boride containing layers, or borides applied to hardmetal cutting tools, have not enjoyed widespread commercial use. This is surprising in view of the many attempts undertaken to utilize bronoic phases to provide surface protection for hardmetal components subject to wear.
For example, a 1977 French paper (preprints of the 9th PLANSEE Seminar, held May 23-26, 1977) reported titanium boron nitrides deposited onto hardmetals. Compared to TiN coatings of the same thickness (about 10 um), the materials of that report had a tool life clearance face wear resistance two to three times greater than TiN coated materials. The report, however, does not recite results on their resistance against impacts (interrupted cutting) and hence it cannot be determined whether the samples had the toughness required for today's applications. The CVD layers reported therein were generated at temperatures ranging from 1150.degree. and 1450.degree. C. and provided a two-phase composition comprised of a mixture of titanium diboride and titanium boronitride. According to the results reported therein, diborides were formed in a second phase--perhaps as a result of reduced solubility of boron in titanium nitride. Decreased boron in the gaseous mixture during the CVD process did not change this result. According to the Paper, a brittle tungsten cobalt boron phase (WCoB) was formed in the external zone of the WC-Co hardmetals, resulting in a serious disadvantage.
Specification JP 58-67858 proposes applying an aluminum oxide layer to a hardmetal and a TiC intermediate coating wherein the aluminum oxide layer is sandwiched between a Ti(B,N) layer. The boron content in the boronitride coating therein is 5-40 mol%, and the Ti(B,N) layer is formed in two-phases, as titanium diboride and titanium boronitride. The borides in this teaching function to improve layer adhesion.
U.S. Pat. No. 4,239,536 teaches a coated hardmetal having at least one layer of metal-boronitride, and boron carbonitrides, of the elements of subgroups 4 to 6 of the periodic table. These diborides are said to confer increased wear resistance for cutting inserts in machining. But because diborides react with ferrous materials considerably stronger than nitrides, a boride nitride mixed phase is used, having a maximum 50% boron mol content. Comparative tests described in the patent show that inserts with the two-phase layers therein (Ti(B,N)+TiB.sub.2) exhibit longer tool life than single-phase layers (Ti(B,N)), and that optimal tool lives are achieved from boron nitride layers having a 30-30% TiB.sub.2 content. The formation of a brittle tungsten cobalt boron external zone in WC-Co hardmetals is compulsory in such instances.
At the 1981 PLANSEE Seminar (in: Proceedings of the 10th PLANSEE Seminar 1981, Vol. 1, pp. 443 ff.), Zemann ct al. reported the deposition of TiB.sub.2 layers onto hardmetals from the gaseous phase. Deposition occurred at temperatures ranging from 700.degree.-1070.degree. C. using BBr.sub.3 to introduce boron. Widely varying rates of deposition and layer structuring (depending on the coating conditions selected) were reported. The boron produced a brittle tungsten cobalt boron outer phase. The deposition of an intermediate TiC layer is reported to give protection against formation of a tungsten cobalt-boron phase in the hardmetal random zone.
Specification DE-OS 33 32 260 discloses a very economic formation of tungsten cobalt boron phase on the surface of WC/Co hardmetals. The WC/Co hardmetal is treated with boron prior to oxide layers deposition resulting in the generation of the hard and brittle tungsten cobalt boron phase.
A coated hardmetal is also described in specification EU 0 015 451 wherein boron, silica or aluminum are diffused into the hardmetal. Initially, one or more layers of carbides, nitrides, or carbonitrides are applied. Then, a coating of titanium, hafnium, zirconium or tantalum boride or diboride is applied as a protective layer.
U.S. Pat. No. 4,236,926 discloses increased wear resistance via boronization without additional deposition of coatings. The hardmetal is exposed to a gas mixture of boron trihalogenide and hydrogen halogenide at temperatures ranging from 715.degree. and 1000.degree. C.
Other references have proposed using boron. For example, AT-PS 377 786 teaches providing an aluminum oxide layer containing small amounts of boron as a protective, wear-resistant layer for hardmetals. DD-PS 225 454 discloses boride layers applied to ferrous materials. DE-OS 35 02 262 discloses a gaseous, boronic atmosphere for the manufacture of boronic layers using PVD and CVD processes; and, JP-OS 61-177372 teaches production of boron nitride films.
The preceding publications show that many attempts have been undertaken to utilize borides as wear-resistant materials. The diverse nature of the layering conditions utilized, contrasting methods, different goals, structures, materials, compositions, and different layering sequences make it apparent that a single advantageous technique still has not been developed to effectively utilize the advantages borides offer as protective wear-resistant layers. The art still is using carbide- and/or nitride-coated hardmetal inserts in largest measure.